Conventionally, a hybrid drive device has been proposed as shown in the Patent Literature 1, which is formed by an engine, a clutch connecting or disconnecting the output shaft of the engine and an input shaft of the gear mechanism and a motor/generator rotating in association with the rotation of the input shaft. According to this hybrid drive device shown in the Patent Literature 1, the engine is restarted from the engine being in a stopped state by gradually increasing the engine rotation speed by gradually transmitting the rotation drive force from the motor/generator to the engine by gradually connecting the clutch which has been in a disconnected state.